The "miles to empty" displayed by the computer is based on your average MPG stored in the computer. I do not know if in Volvos it is the average since the last time the average MPG was reset, or a "smarter" average based on the last XX miles driven, as in some other cars with OBCs.

Since my computer average MPG usually shows 12-13 mpg, the reserve light comes on at 20 or 25 miles to empty (it only reads in 5 miles increments).

The above mileage figures, kind of low, are for our case: doing mostly city driving, and living in an area in which neighbors are asking (and getting) that a 4-way stop sign be installed in every possible corner, in the whole city. So my usual driving pattern is accelerate for 2/3 of a block, and brake for 1/3 of a block, almost every block of my drive. Not only a big waste of gas in non-hybrid cars, but a very slow way of getting to your destination.....

Andy
98 V70-T5